Why does my URL get processed before I press Enter?

I'm working through Adam Freeman's book "Pro ASP.Net Core 3" (8th edition). I am running an example of a page that uses information in a database. The Configure() method in my Startup class includes an endpoint for "/sum/{count:1000000000}". The base URL is http://localhost:5000. In Google Chrome, I type "http://localhost:5000/sum", and as soon as I type the "m", the web page is requested. If I want to get the calculation for 10 by requesting http://localhost:5000/sum/10, the page for 1000000000 is requested first. I can imagine that in a real-world application, that could end up being a problem. How do I avoid that?

Here's my startup file:

using Microsoft.AspNetCore.Builder;
using Microsoft.AspNetCore.Http;
using Microsoft.Extensions.Configuration;
using Microsoft.Extensions.DependencyInjection;
using Platform.Services;
using Microsoft.EntityFrameworkCore;
using Platform.Models;
using Microsoft.Extensions.Hosting;
using Microsoft.AspNetCore.Hosting;

namespace Platform {
    public class Startup {

        public Startup(IConfiguration config) {
            Configuration = config;
        }

        private IConfiguration Configuration { get; set; }

        public void ConfigureServices(IServiceCollection services) {
            services.AddDistributedSqlServerCache(opts => 
            {
                opts.ConnectionString
                    = Configuration["ConnectionStrings:CacheConnection"];
                opts.SchemaName = "dbo";
                opts.TableName = "DataCache";
            });
            services.AddResponseCaching();
            services.AddSingleton<IResponseFormatter, HtmlResponseFormatter>();

            services.AddDbContext<CalculationContext>(opts =>
            {
                opts.UseSqlServer(Configuration["ConnectionStrings:CalcConnection"]);
            });
            services.AddTransient<SeedData>();
        }

        public void Configure(IApplicationBuilder app,
                IHostApplicationLifetime lifetime, IWebHostEnvironment env,
                SeedData seedData) {
            app.UseDeveloperExceptionPage();
            app.UseResponseCaching();
            app.UseStaticFiles();
            app.UseRouting();
            app.UseEndpoints(endpoints => {

                endpoints.MapEndpoint<SumEndpoint>("/sum/{count:int=1000000000}");
                endpoints.MapGet("/", async context => {
                    await context.Response.WriteAsync("Hello World!");
                });
            });

            bool cmdLineInit = (Configuration["INITDB"] ?? "false") == "true";
             if (env.IsDevelopment() || cmdLineInit) {
                seedData.SeedDatabase();
                if (cmdLineInit) {
                    lifetime.StopApplication();
                }
            }
        }
    }
}